An image forming apparatus using columnar toner has been proposed conventionally. The columnar toner is advantageous in that the toner has uniform diameter easily, the toner can be produced with a high yield, and the toner forms an image with even thickness easily. With these advantageous properties, the columnar toner is expected to attain (a) stable image density, (b) better control over color density (gradation), (c) higher resolution, (d) prevention of photographic fog and or prevention of dusts (dots in white space) in transferring, (e) prevention of filming a developing roller or photoreceptor with toner particles, and the other effects.
For example, Patent Document 1 (Japanese Patent Application Publication, Tokukai, No. 2006-106236 (published on Apr. 20, 2006) discloses an art of producing columnar toner particles, the art including extruding a molten toner material into a fiber shape via a nozzle, cutting the fiber-shaped toner material thereby forming cylindrical particles, binding an external additive to surfaces of the cylindrical particles by applying a mechanical force on a mixture of the cylindrical particle and the external additive, and chamfering edges of the cylindrical particles.
Patent Document 2 (Japanese Patent Application Publication, Tokukai, No. 2000-122442 (published on Apr. 28, 2000) discloses an art in which a rotating rate of a transfer-giving (member from which the toner image is transferred) is slightly faster than that of an image transfer body (member for receiving the transfer), or a moving speed of a recording medium. In a conventional electrophotographic image forming apparatus, this art makes it easy to remove toner particles from the transferring-giving member.
Patent Document 3 (Japanese Patent Application Publication, Tokukai, No. 2003-149955 (published on May 21, 2003) discloses an art for improving resolution in a sub-scanning direction. In the art, an electrostatic latent image pattern is formed on a surface of an electrostatic latent image bearing member, the electrostatic latent image pattern being extended along the sub-scanning direction at an A/B ratio with respect to an original image pattern, and the electrostatic latent image bearing member being rotated at a speed of Vopc, which is faster than a device process speed Vpro (Vopc is faster than Vpro by B/A times). Then, the extended electrostatic latent image pattern is visualized by developing means. The visualized image is transferred to an intermediate transfer body rotating at the device process speed Vpro, thereby forming an image pattern in a desired size.
Moreover, Patent Document 4 (Japanese Patent Application Publication, Tokukaihei, No. 4-86878 (published on Mar. 19, 1992) discloses an art for preventing toner coagulation between a transfer nip thereby attaining good-quality printing free from blank of a line or a character in a recording apparatus having an image bearing member for forming a toner image thereon and, transfer means for transferring a toner image to a toner image receiving member from the image bearing member. The toner coagulation is prevented by applying an AC transfer bias of a frequency that satisfies Fr 4000 and (Fr×d)/V≦Fr(Hz) where Fr(Hz) is a frequency of an AC bias, d (mm) is a nip distance between the transfer means and the image bearing member, and V(mm/sec) is a moving speed of the image bearing member.
In such an image forming apparatus using the cylinder toner, the image is formed with toner particles whose longitudinal directions are oriented randomly in various directions including a main scanning direction and the sub scanning direction. This would result in inconsistency of density due to spaces formed between the randomly-oriented toner particles, or would cause scattering of the toner particles.
FIG. 7 is an explanatory view schematically illustrating how a toner image developed on a photosensitive drum 101 is transferred to an intermediate transfer belt 102 in an image forming apparatus using conventional columnar toner. As illustrated in FIG. 7, the intermediate transfer belt 102 is pressed against the photosensitive drum 101 by a transfer roller 103, thereby forming a transfer nip section, at which the toner image is transferred from the photosensitive drum 101 to the intermediate transfer belt 102. In the image forming apparatus using the conventional columnar toner, the photoreceptor drum 101 and the intermediate transfer belt 102 are rotated at such speeds that they move at equal velocity at the nip position where they are in contact with each other.
As illustrated in FIG. 7, the toner image developed on the photosensitive drum 101 is formed from toners whose longitudinal directions are randomly oriented. This cases gaps between toner particles within a dot (between the toner particles contributing the image formation). Thus, the toner particles cannot stably in touch with each other. This would cause scattering of toner before fixation, or inconsistency of density after the fixation.
The arts disclosed in Patent Documents 2 and 3 are expected to be effective to easily remove the toner particles from the member from which the toner image is to be transferred. However, they do not consider the use of the columnar toner and cannot enjoy the advantages (a) to (e) provided by the use of the columnar toner in the image forming apparatus.
Moreover, the art disclosed in Patent Document 4 is expected to be effective in preventing blank of lines or characters during the image transfer. However, it does not consider the use of the columnar toner and cannot enjoy the advantages (a) to (e) provided by the use of the columnar toner in the image forming apparatus.